Journal of Optics (United Kingdom) (20408986)26(8)
In recent years, extracting information from superposed vortex beams has been a topic of intense study. In this paper, complex coefficients of various superpositions are measured in both simulation and experiment by proposing and implementing four different sampling methods. Superposed vortex beams are experimentally generated using a digital micromirror device, and recorded on a 2 f optical imaging setup. To extract both amplitude and phase values of modal coefficients, a single intensity frame of the beam is sampled in the form of concentric circles, sectors, random circles, and random squares. Considering just specified parts of the intensity instead of the whole to sample the pattern increases the speed of the modal coefficient extraction. Besides, a linear set of coherent equations is solved, and achievements are compared together. As a consequence, measuring both the amplitude and phase values of coefficients simultaneously can pave the way to enable high-capacity optical communication which is carried out in this research with better than 99% and 96% accuracy, respectively. © 2024 IOP Publishing Ltd.
Optical and Quantum Electronics (discontinued) (03068919)55(6)
A theoretical model is discussed to investigate the impact of a gas-filled hollow-core photonic crystal fiber (HC-PCF) on the temporal shape and width of a picosecond input pulse. For this purpose the nonlinear Schrödinger equation is numerically solved to simulate the evolution of a slowly varying approximated soliton propagating along the waveguide. To obtain a highly desirable pulse shape with the shortest width at the output, four fiber schemes proposed as case-I to case-IV HC-PCF are modelled in which the pressure style of the filler gas and the chirp of the input pulse are purposefully engineered. The well-known distance parameter and the Q factor are used to evaluate the temporal feature of the ultimate pulse. The final study on the compressed pulse characteristics is performed using a standard merit function (SMF) that is introduced to obtain the most optimum output pulse shape and width. It is found that the application of a positive gradient for the gas pressure and an adequate chirp value for the input makes it possible to reach the lowest width and the highest temporal quality. The final results suggest that the cascaded case-IV fiber is capable of generating the most optimal pulse width of 18.1 fs with the highest SMF value of 12, indicating the highest temporal quality and the lowest distance parameter of 46.3% and 194, respectively. The most interesting feature of the results is the fact that by the proper use of a gradient from for the gas pressure the effect of chirp can be directed in a way that the highest temporal quality and the most desired width of compressed pulse obtains while the input power is kept low. © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
Journal of Optics (United Kingdom) (20408986)25(4)
The phase angle of the vortex beam along a closed loop centered on the optical singularity changes by 2πℓ, where ℓ is the number of phase jumps (PJs) from 0 to 2π and indicates the topological charge of the vortex beam. In this paper, the generation and specification of a new type of vortex beam, in which their PJs are asymmetrically embedded in the phase pattern, are reported. In contrast to Laguerre-Gaussian vortex beams, where PJs are equally spaced azimuthally around the optical singularity, the presented vortex beams have PJs embedded at arbitrary azimuthal angles. By designing a particular forked grating and displaying it on a spatial light modulator, this type of vortex beam is experimentally generated. As with conventional forked grating, the designed grating produces vortex beams with the opposite orbital angular momentum (OAM) sign in the first diffraction order. By measuring the relative orientation of the intensity profile of these OAM beams in the first diffraction order, the position of the PJs on the wavefront of a vortex beam with ℓ = 2 can be determined. This type of vortex beam could have potential applications in various fields of photonics, especially in optical communications based on optical vortices. © 2023 IOP Publishing Ltd.
Optics Express (10944087)31(17)pp. 28459-28469
Diffraction patterns of optical vortex beams (VBs) by differently shaped apertures are used to determine their topological charge (TC). In this paper, we show by simulations and experiments that diffraction of a Laguerre-Gaussian (LG) beam by intersecting circular apertures can be used to reveal the TC. The presented aperture structure has the advantage of the measurement of fractional TC in addition to the integer, sensitivity to the sign of TC, and low sensitivity to adjusting apertures. Accordingly, in addition to the integer TC up to 8, the fractional TC is measured with a step of 0.1 by two intersecting circular apertures (TICA). By examining a wide range of similarity criteria between the diffraction pattern of the fractional TC and the pattern of the lower integer TC, three metrics for measuring the fractional TC are found. Furthermore, the determination of integer TC up to 6 for three intersecting circular apertures (THICA) is demonstrated. © 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement.
Applied Optics (21553165)62(13)pp. 3409-3415
Measurement of the topological charge (TC) of vortex beams, including integer and fractional orbital angular momentum, is of great importance in diverse fields. Here we first investigate the diffraction patterns of a vortex beam from crossed blades with different opening angles and positionings on the beam by a simulation and experiment. Then the positions and opening angles of the crossed blades that are sensitive to the variation of TC are selected and characterized. We show that for a specific position of the crossed blades on the vortex beam, the integer TC can be measured directly by counting the bright spots in the diffraction pattern. Moreover, we show experimentally that for other positions of the crossed blades, by calculating the first-order moment of the intensity of the diffraction pattern, the integer TC between −10 and 10 can be obtained. In addition, this method is used to measure the fractional TC and, as an example, the TC measurement is demonstrated for a range between 1 and 2 with 0.1 steps. The result of the simulation and experiment shows good agreement. © 2023 OSA - The Optical Society. All rights reserved.
Optics and Laser Technology (00303992)157
We demonstrate the effect of the resolution of forked-diffractive optical elements (DOEs) on the helicity of the topological charge (TC) of the orbital angular momentum (OAM) beams. To aim that, the forked-DOEs are designed with the same TC while they are different in resolution. Changing the resolution allows the fork direction to be switched. Interestingly, switching the sign of the TC is the consequential achievement for the OAM beams, generated by the fork grating elements with specific resolutions. Furthermore, changing the resolution make the fork gratings to be designed for generating petal-like beams in which the magnitude of the TC tends to zero. Finally, three criteria are calculated for all intensity distributions to quantify the generated beams’ quality and investigate where the helicity of the TC changes. Consequently, we demonstrate the emphasis on selecting the resolution of DOEs which plays a pivotal role in optical systems, for instance in optical tweezers where the OAM beams need to be generated with a desired sign of TC. © 2022 Elsevier Ltd
Optics Express (10944087)30(6)pp. 9494-9504
We propose the use of an intensity technique to decompose superpositions consisting of two, three, or four basis Laguerre-Gaussian (LG) modes, and measure the orbital angular momentum (OAM) of such superpositions. The mode generation and decomposition are both accomplished only on a 2f optical imaging system. We demonstrate numerically and experimentally that the squared amplitudes of superpositions can be determined by recording a single frame of the intensity distribution. This is accomplished by measuring the intensity along certain circles and solving a linear set of equations relating the sampled intensities to squared amplitudes. The accuracy of better than 98% for composite beams consisting of two, and about 90% for composite beams consisting of more than two basis modes are achieved. Finally, we report the value of the measured OAM of the superpositions with excellent accuracy regarding theoretical values, for small and large integer and non-integer OAM. © 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Optics Express (10944087)30(13)pp. 23025-23034
The Airy beam is the solution of Maxwell's wave equation and since this equation is linear, a superposition of Airy beams still remains the solution of the wave equation. In this paper, we propose a method for generating multiple Airy beams that includes a desirable number of up to 6 individual Airy beams with desirable acceleration properties. By introducing a decenter into the designed diffractive optical element (DOE) of an Airy beam the problem of generating dual airy beams patterns by an amplitude-based spatial light modulator is solved. By superimposing the designed DOEs of individual Airy beams and scaling them to the proper gray level range, the DOE of the multiple Airy beams is generated. Displaying this DOE on a digital micromirror device, multiple Airy beams are experimentally produced. The experimental studies of these beams show good agreement with the performed simulations. © 2022 Optica Publishing Group.
Optical and Quantum Electronics (discontinued) (03068919)54(2)
We report on the nonlinear properties of size-variable Silver nanoparticles (AgNPs) which are synthesized based on the complexing agent method using different pH of the synthesizer. The z-scan technique is used to characterize nonlinear optical properties of AgNPs products. The required z-scan sample is provided by suspending the AgNPs in distilled water and dispersing them in Potassium Bromide (KBr) as host materials. Obtained results indicated an average nonlinear index of − 0.79 × 10–7 cm2/W in water and of − 2.09 × 10–7 cm2/W in KBr at 1064 nm. Furthermore, an average nonlinear absorption of 2.23 × 10–3 cm/W in water and 5.09 × 10–3 cm/W in KBr is measured for the synthesized AgPNs. The results affirm that as the AgNPs sizes are increased, their nonlinear properties are significantly improved in both KBr and water. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
Journal of Optics (United Kingdom) (20408986)24(6)
The manipulation of the orbital angular momentum (OAM) spectrum in new wavelengths generated by frequency conversion has a wide range of applications in quantum information technology and modern communications. Here, the second harmonic generation of a vortex beam in a nonlinear crystal with an inhomogeneous refractive index is considered. It is shown that by using a segmented oven and generating a controllable temperature gradient along the thickness of the nonlinear crystal, a transverse phase mismatch can be induced in the vortex beam. Accordingly, the OAM spectrum of the vortex beam can be changed by generating new OAM components in the spectrum. Compared with the nonlinear Bragg diffraction method, the presented method has the advantage of longer interaction length and higher efficiency. © 2022 IOP Publishing Ltd.
Iranian Journal of Physics Research (16826957)22(3)pp. 629-638
Laser beams carrying orbital angular momentum have found numerous applications in various branches of sciences especially in modern quantum communications. Frequency conversion of these beams using nonlinear crystals allows the desired spectral ranges to be achieved. In this work, first, the optimal value of the focusing parameter and the location of the focus of the pump beam with orbital angular momentum in second harmonic generation process are determined. Then, based on the dependence of the optimal value of the phase mismatch parameter on the Laguerre-Gaussian mode order contained in the pump beam, a novel method for varying of the orbital angular momentum of the generated second harmonic beam is presented. According to this method, for pump beams with fractional orbital angular momentum, by changing the focusing parameter, the angular momentum of the second harmonic generation beam can be continuously tuned. © 2022, Isfahan University of Technology. All rights reserved.
Journal of Optics (United Kingdom) (20408986)23(12)
In this work, a theoretical model is discussed to investigate the performance of a core-engineered gas-filled hollow-core photonic crystal fiber (HC-PCF). To gain the shortest pulse width and the best beam quality at the output, the core geometry of the fiber is modified within four specific types referred to as type I to type IV fibers. It is found that, by using type III and type IV HC-PCF devices, a 5 ps laser pulse in the input can be respectively compressed to 18.5 fs and 13.7 fs at the output. It is found that, a 5-ps laser pulse in the input can be reduced to 18.5 fs and 13.7 fs if type III- and type IV of modified HC-PCF device are respectively used for compression. The structural similarity (SSIM) index is used to evaluate the quality of the beam cross-section that ultimately emerges from the end of the fiber. The results suggest that the highest SSIM value of 0.76 can be obtained if type III HC-PCF is employed for pulse compression. © 2021 IOP Publishing Ltd.
Applied Physics B: Lasers and Optics (09462171)127(9)
A multi-segment oven scheme is proposed to control the temperature distribution inside a MgO:PPLN crystal to generate a second-harmonic generation (SHG) pulse possessing optimum characteristics. It is found by numerical solving of SHG coupled equations and steady-state heat equation that when the oven is divided into four segments, the most optimal pulse features can be reached. While the temperature of each segment is independently set at a certain value, and the input pump is raised to 103 GW/m2, the lowest distance parameter of about 10 and the highest efficiency of around 85% is entirely expectable, indicating a desirable case whereby keeping the crystal at the phase-matching condition could not be reached. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.
Journal of the Optical Society of America B: Optical Physics (07403224)38(9)pp. 2654-2662
The fundamental beam wavefront shaping method is developed to increase the nonlinear frequency convention efficiency and control of the nonlinear beam profile. In this work, a method is presented to accelerate the procedure of optimizing the wavefront phase of the fundamental beam to approach the maximum second-harmonic generation (SHG) efficiency. Furthermore, this method allows use of high-resolution spatial phase modulators for wavefront shaping while the number of phase optimization variables remains limited. The obtained results show that the presented method reduces the amount of the needed feedback by 3 orders of magnitudes while the SHG efficiency is improved twice. © 2021 Optical Society of America
Journal of the Optical Society of America B: Optical Physics (07403224)38(1)pp. 30-35
We represent a detailed design of a plasmonic refractive index sensor based on a metal-insulator-metal Bragg microcavity in which two simultaneous mechanisms boost the sensing performance: high field confinement and resonant assisted multiple-pass of the plasmonic waves. We investigate in detail the effect of structural parameters on the plasmonic spectral properties such as depth of dip and spectral width since these parameters determine the sensor performance such as sensitivity, resolution, and signal-to-noise ratio and thus have a great significance in sensor ability. We explain the physical reasons for all of the observed behavior in the proposed sensing structure. © 2021 Optical Society of America
Applied Optics (21553165)60(31)pp. 9728-9735
We provide an open-source user-friendly graphical-user interface software in a MATLAB environment, named Speckle Analyzer, as a tool for calculating and analyzing statistical parameters of a laser speckle pattern to find metrics for an object’s physical quantity. The first- and second-order statistical functions containing gray-level co-occurrence and gray-level run-length matrices and speckle grains geometrical properties are included in Speckle Analyzer. To validate the software’s operation, statistical parameters of the laser speckle pattern, to find metrics for the size and concentration of particles suspended in liquid, are investigated. © 2021 Optical Society of America
Journal of the Optical Society of America A: Optics and Image Science, and Vision (15208532)38(1)pp. 1-9
We report on the quality assessment of an optical coherence tomography (OCT) image. A set of recent digital filters are used for denoising the interferometric signals. It is found that when a combination of continuous wavelet transform (WT) decomposition and the WT denoising techniques is imposed on raw signals, the highest signal-to-noise ratio of 17.8 can be reached. The structural similarity (SSIM) index is eventually employed to evaluate the modality of the reconstructed OCT image. Further, we found out that a SSIM value of about 0.95 can be reached, independent of the method used for envelope extraction. © 2020 Optical Society of America
Journal Of Theoretical And Applied Physics (22517227)14(4)pp. 399-409
The performance of an apodized gas sensor is demonstrated through simultaneous detection of CO and CO2 absorption lines around 1.57 µm in the recuperator channel of a gas-fired industrial furnace at Shahid Montazeri power plant (SMPP) industry. This led to the concentration measurement of targeted molecules as less than ~ 1% and 9.5%, respectively, at atmospheric pressure and 350 °C, indicating close consistency with the reference data reported by SMPP. A minimum detectable absorption of ~ 0.4 × 10−3, corresponding to a detection sensitivity of ~ 4.8 × 10−9 cm−1 Hz−1/2 is measured in this application. © 2020, Islamic Azad University.
Journal of the Optical Society of America B: Optical Physics (07403224)37(1)pp. 9-18
The performance of a double-pass scheme is theoretically investigated for the efficiency enhancement of a second-harmonic (SH) beam generated by using a high-power fundamental laser beam. Based on a modified version of coupled equations that include the possible effects leading to the thermal de-phasing, the Collins integral is used for optimizing the focusing optics to obtain maximum efficiency of SH conversion. We found through simulation that at a fundamental power of 40 W, when the focusing regime is set for the loose condition, a conversion efficiency of 74% can be reached. It is possible if the focusing points of the first and second passes are designed to be located at 19 mm and 6 mm away from the input face of the crystal, respectively. © 2019 Optical Society of America
Journal of Optics (United Kingdom) (20408986)22(6)
Providing the phase-matching condition is crucial to achieve efficient frequency conversion in a nonlinear crystal. Birefringence and periodically poled nonlinear crystals are widely used to establish the phase-matching condition. Here, a method is proposed to overcome the phase mismatching between interacting waves in a nonlinear crystal by modulation of the fundamental beam intensity and phase. Accordingly, the fundamental beam is focused by a multi-focal length diffractive optical element into a nonlinear crystal. Therefore, the fundamental beam suffers a specified number of foci along the propagation direction. The focus points of the fundamental beam are adjusted to coincide with locations that the interacting waves are in phase and for the length that the phases of the waves are opposite, the fundamental beam has a low intensity. Consequently, the back conversion is partially suppressed, and a net of conversion efficiency can be achieved. The method is validated by simulation of the second harmonic generation process. © 2020 IOP Publishing Ltd.
Optics Express (10944087)28(14)pp. 20523-20531
Plasmonic electro-optic modulators might play a pivotal role in the development of compact efficient communication devices. Here, we introduce a novel electro-optic modulator based on a plasmonic Bragg microcavity and a pockels active material. We investigate detailed design and optimization protocols of the proposed structure. With 2D scanning of geometrical parameters, an extinction ratio of 19.8 dB, insertion loss of 2.8 dB and modulation depth of 0.99 with a driving voltage of ±5 V are obtained. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Applied Physics B: Lasers and Optics (09462171)126(5)
We introduced a theoretical model to preserve the quality and the efficiency of a picosecond second-harmonic generation (SHG) in a 30-mm-long LBO crystal which is surrounded by a thermal oven. It is proposed to arrange a temperature gradient throughout the crystal length for mitigating the undesirable consequences of dispersion and group velocity mismatch. To optimize the pulse shape, a distance parameter is introduced and in addition, the SHG efficiency and pulsewidth are included in a merit function. By setting the temperature gradient as the variable the merit function is optimized. The results show that for temperature gradient of 60 °C, an SHG pulse with mostly Gaussian shape with 80.7% efficiency and with 4.47 ps pulsewidth is attainable. Furthermore, the effect of the chirping on the SHG pulse is investigated. Final results have turned the proposed model into an excellent candidate for obtaining an efficient SHG conversion with an appreciable pulse quality. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.
Applied Optics (21553165)58(27)pp. 7531-7537
In this paper, a method is proposed to design a binary multifocal Fresnel zone plate (MFZP) with desirable focal lengths and number of foci. By performing simulations, the focusing characteristics of MFZPs with two, three, and four focal lengths are investigated. The obtained results show that the aspect ratio of focal lengths and the number of foci of MFZPs can be chosen freely. Furthermore, designed MFZPs are displayed on a digital micromirror device, and the focusing properties of the reflected laser beam are investigated. Experimental results support the validity of the method. © 2019 Optical Society of America.
Journal of Optics (United Kingdom) (20408986)21(11)
This work provides a comprehensive study around the effect of a quality-deteriorated fundamental beam on the quality of a 355 nm wavelength beam experimentally generated using sum-frequency mixing of a second-harmonic generation (SHG) beam and its un-depleted fundamental inside a walk-off (WO) affected BBO nonlinear crystal. A slit-variable aperture is used inside the resonator of a utilized Nd:YAG resonator to generate a wide range of Hermite-Gaussian modes for the fundamental and subsequent SHG and sum-frequency generation (SFG) beams. The M2-factor of interacting beams is simulated based on the reconstruction of the recorded intensities using a recently reported technique known as few-frame decomposition method. It is found that in the worst case, due to the WO occurring in the X-direction, the M2-factor of the SFG beam is converged to 2.5 after 2 mm of crystal length and remained constant in the remaining length when the fundamental M2-factor is set for 4.5. Based on the simulation results, it is shown that the beam quality of SFG beam along the WO direction is independent of the fundamental M2-factor. It is further realized that by changing the focusing factor up to 2.84, again M2-factor of the UV-SFG beam is not exceeded from 2.5. © 2019 IOP Publishing Ltd.
Optical and Quantum Electronics (discontinued) (03068919)50(10)
We report on the capability of adaptive neuro fuzzy inference system (ANFIS) coupled with discrete wavelet transform (DWT) for the de-noising of absorption signals. The proposed method is examined for the de-noising of a CO2 spectrum which is experimentally traced in the near-infrared around 1.57 μm. This is performed by contaminating the spectrum with a various types of complicated noise figures produced by several approaches. It is found that DWT-assisted ANFIS is quite sufficient for CO2 signal recovery that is obscured by a huge amount of unknown noises without need of using a prior estimation of the noise feature. Eventually, enhanced signal-to-noise ratio of about 11.7 is obtained. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
Iranian Journal of Physics Research (16826957)18(2)pp. 207-220
An annular cell with elliptical cross section and small physical size is introduced and simulated for increasing the absorption length in the spectroscopy. In this investigation by changing of the geometrical radius of the ellipse an absorption length of 15.50 m is obtained. By the optimization of the cell and including the sagittal and tangential radius of the ellipse that reflect the beam, it is shown that at the obtained absorption length, it is possible to reduce the divergence of the beam down to 54 µm. Then, signal-to-noise ratio (SNR) is calculated for R(16) CO2 absorption line by assuming that the cell is filled by the gas. The results of this calculation indicated that under optimum condition the SNR can be increased up to 310. Finally, the cell is optimized using Genetic algorithm by including all the effective parameters which affect the efficiency of the cell. We found that in the best case an absorption length of 8.24 m with a SNR of 107 can be achieved when the geometrical radius of the cell being fixed at 5.95 and 4.59 cm. © 2018, Isfahan University of Technology. All rights reserved.
IEEE Photonics Technology Letters (10411135)30(13)pp. 1186-1189
We constructed a relatively simple inexpensive wavemeter with completely static parts to measure the laser wavelength accurately based on diffraction from a Fresnel zone plate (FZP) displayed on a digital micromirror device. The unknown laser wavelength was measured by finding the proper FZP, focusing the laser beam exactly into the specified focal point. The relative uncertainty in the order of 10-5 was attained in He:Ne and second harmonic of Nd:YAG laser wavelength measurement. © 1989-2012 IEEE.
Journal of Optics (United Kingdom) (20408986)20(4)
We present a theoretical model in order to study the effect of a thermally loaded crystal on the quality of a second-harmonic (SH) beam generated in a high-power pumping regime. The model is provided based on using a particular structure of oven considered for MgO:PPsLT nonlinear crystal to compensate for the thermal de-phasing effect that as the pumping power reaches up to 50 W degrades the conversion efficiency and beam quality of the interacting beams. Hereupon, the quality of fundamental beam is involved in the modeling to investigate the final effect on the beam quality of generated SH beam. Beam quality evaluation is subsequently simulated using Hermite-Gaussian modal decomposition approach for a range of fundamental beam qualities varied from 1 to 3 and for different levels of input powers. To provide a meaningful comparison numerical simulation is correlated with real data deduced from a high-power SH generation (SHG) experimental device. It is found that when using the open-top oven scheme and fixing the fundamental M 2-factor at nearly 1, for a range of input powers changing from 15 to 30 W, the M 2-factor of SHG beam is degraded from 9% to 24%, respectively, confirming very good consistency with the reported experimental results. © 2018 IOP Publishing Ltd.
Applied Optics (21553165)57(30)pp. 9140-9147
We report on the performance of an enhanced-cavity (EC) designed for obtaining high-power and efficiency second-harmonic generation (SHG). This is performed by numerical simulation of SHG coupled equations in the presence of a thermal dephasing effect that is effectively intensified through embedding the periodically poled LiNbO3 crystal by an oven-surrounded scheme. It is found that by setting the PPLN temperature at an optimum value, adjusting the mirror reflectively, and pumping power at certain values, gaining SHG efficiency of more than 90% is possible. We further realized that by an ECSHG device SHG efficiency can be improved by about 15%-50%. Moreover, compared to a single-pass SHG scheme, the EC-based SHG device is shown to be a very promising candidate to reduce and suppress the effect of thermal dephasing on the stability and efficiency of SHG radiation. © 2018 Optical Society of America.
Applied Physics B: Lasers and Optics (09462171)123(9)
An innovative optical method is introduced for the beam quality measurement of any arbitrary transverse mode based on the reconstruction of the mode from a few-frame image of the beam cross-section. This is performed by the decomposition of a mode to its basic Hermite–Gaussian modal coefficients. The performance of the proposed method is examined through M2-factor measurement of the beam of a Nd:YAG laser which was forced to oscillate in a certain mode using a crossed rectangular intracavity aperture. Obtained results have shown that this method can be alternatively replaced for the hologram- and ISO-based techniques recently exploiting for beam quality measurement regardless of the mode type and the position of utilized CCD camera along the beam direction. © 2017, Springer-Verlag GmbH Germany.
Optical and Quantum Electronics (discontinued) (03068919)49(11)
In the present work, the effects of an intracavity etalon inside a singly resonant optical parametric oscillator (SRO-OPO) resonator on the power and spectrum stability and tuning characteristics of the signal and idler waves has been theoretically modeled. Real experimental parameters associated with a laboratory SRO-OPO are used to compute the spectral bandwidth of oscillating signal through calculation of the nonlinear coupled equations while the input pump power is assumed to be adjusted at several multiples of oscillation thresholds for sets of thickness-variable slab glass etalon. Simulation results show by using thicker etalons a significant reduction of ~ 60 times in the signal bandwidth can be obtained at higher levels of the SRO-OPO threshold which resulted in the more stability of the outcoupled waves. The simulated results have been realized by using a slab glass of etalon inside the cavity of a PPLN-base SRO-OPO setup constructed in the laboratory. It is found that the peak-to-peak fluctuations of both signal and idler waves reduces to less than ~ 0.05% whereas the thickness of the internal etalon is increased to ~ 1.1 mm and the device is adjusted to operate around the threshold. This reflects good consistency between experimental measurements and theoretical model. Finally, more than ~ ± 15 nm tuning of the SRO-OPO output is also performed by rotating the intracavity over ~ ± 5°. © 2017, Springer Science+Business Media, LLC.
Optical and Quantum Electronics (discontinued) (03068919)48(4)
The potential and drawbacks of the apodized 2f/1f wavelength modulation spectroscopy are theoretically studied and experimentally characterized. We apply a near-infrared DFB-based laser spectrometer, tunable around the R(32) CO2 absorption line centered at 6369.408 cm−1. The performance of the apodized method is shown by minimizing the pressure deviation between the gauge and experimental pressures by using the beneficial effect of the scaling (Formula presented.) -factor. This factor equalizes the experimental and simulated peak heights of the CO2 absorption trace. We found that when (Formula presented.) -factor is varied up to its optimum value of ~200, a pressure deviation of nearly zero is obtained at a case pressure of 19 ± 0.5 mbar. Under such optimum condition a minimum uncertainty of ±1 mbar is also obtained for the pressure deviation. However, we further acquired that far from this optimum condition, compared to the common method, the apodized approach is also capable of reducing the pressure deviation by ~13.5 % at 20 ± 0.5 mbar of CO2 pressure, indicating the performance of the proposed method for precise pressure measurement of a gas sample, regardless of the optical limits. © 2016, Springer Science+Business Media New York.
Optica Applicata (00785466)46(1)pp. 103-115
We report on a multichannel sensor for multipoint corrosion monitoring of an aluminum film, which was corroded by nitric acid as a corrosive solution. The sensor head was a commercial 1×4 optical fiber coupler combined with the optical time domain reflectometry technique. The effects of the concentration and pH of acid on the corrosion rate of the aluminum film are initially studied and characterized. Then, about 100 nm of pure aluminum is coated on the end facets of each fiber channel and reduction of the back reflected signals is simultaneously monitored as a measure of corrosion rate. It is found that the fabricated sensor is very sensitive to the variation of pH as by increasing of the pH up to ~13, the corrosion rate of aluminum is raised to ~75 millimeter per year. Our experimental results have been verified by the standard immersion test, indicating very good consistency and reliability. Eventually, the sensor is used to trace the corrosion of sea water provided from the Persian Gulf, confirming the validity of laboratory results.
Optica Applicata (00785466)46(4)pp. 639-650
In this work, the second-harmonic component of wavelength modulation spectroscopy is simulated for R(22) CO2 absorption line to investigate the effect of gas temperature and pressure on the modulation index. We found that the optimum value of modulation index, that is 2.2, is not affected by temperature but gas pressure will change the optimized modulation index. Specifically, when the gas pressure decreased to lower pressures of less than 100 mbar, the modulation index is also decreased and tended exponentially to about two. Accordingly, the optimum value of modulation index is recalculated for a range of CO2 gas pressures to establish a nearly zero pressure deviation in the spectroscopy of very low pressure samples.
Optical and Quantum Electronics (discontinued) (03068919)48(12)
We applied discrete wavelet transform (DWT) technique for de-noising of carbon monoxide (CO) spectrum which is experimentally obtained in the mid-infrared region using a difference-frequency-based spectrometer. The performance of DWT is firstly examined by de-noising of a simulated P(16) CO absorption line based on the real data associated with the experimental setup we arranged in laboratory. It is found that when the db20 function from Daubechie wavelet family is used at level 7 of composition, highest signal-to-noise ratio (SNR) ~23 can be achieved. Similar results are obtained in the experiment in which a noisy CO trace is de-noised using the same procedure as described in the simulation. Subsequently, highest SNR of ~9.2 and lowest residuals is experimentally obtained using the db20 function which confirms the merit of DWT technique in detection of low-level absorption signals. © 2016, Springer Science+Business Media New York.
Optica Applicata (00785466)46(2)pp. 277-279
In the present work, the inner cladding geometries of a typical double-clad fiber laser are studied and numerically simulated for different cladding shapes to obtain the maximum absorption efficiency for the pump beam. This is performed by using the ray tracing approach and dislocating the fiber core from the center to impose the asymmetry on the investigated geometries. The high absorption efficiency of ∼94.5% was obtained for the optimized offset D-shaped double-clad fiber. The hexagonal shape is proposed as a new geometry for the inner cladding to attain higher absorption efficiency. It was found that the absorption efficiency of ∼ 68% for a symmetrical hexagonal can be improved to ∼95% for an asymmetrized hexagonal-shaped double-clad fiber laser. Eventually the genetic algorithm was used to enhance the performance of the investigated geometries. This resulted in the further increasing of pump beam absorption efficiency of 99.5% in the genetic algorithm optimized asymmetrized hexagonal shape.
Journal of the Optical Society of America B: Optical Physics (07403224)33(12)pp. 1640-1648
In this paper, the effect of thermal dephasing on the efficiency of a high-power and high-repetition-rate singlepass second-harmonic generation (SHG) in the nanosecond regime is studied. We found that inside a thermal loaded crystal, in addition to the optimization of the focusing parameter and oven temperature, spillover loss can also play a controlling role in retaining the SHG efficiency. It is shown that with a proper choice of the focusing parameter and oven temperature, at about 20 W of averaged fundamental power, at least 60% SHG efficiency is achievable. Moreover, exceeding a certain limit of input power up to 80 W, inducing a spillover loss of about 10%, will help to maintain the SHG efficiency at a maximum value of about 50%. © 2016 Optical Society of America.
IEEE Journal of Quantum Electronics (00189197)51(8)
In this paper, we present a theoretical model that describes the performance of a specific oven configuration used to actively control the effect of thermal dephasing on the output characteristics of a high-power single-pass second-harmonic generation (SHG) in a MgO:sPPLT nonlinear crystal. The provided model is based on using step and slope oven configurations and making a quantitative comparison with the recent results obtained by using open-top oven scheme. It is found that in the slope oven scheme the SHG power is enhanced by 190% at a fundamental power of 50 W, indicating the significance of the represented model. Eventually, the merit of the proposed slope oven indicated through saturation control in the generation of high-power SH radiation. © 1965-2012 IEEE.
Optical and Quantum Electronics (discontinued) (03068919)47(10)pp. 3349-3363
We present a theoretical model to investigate the performance of an intracavity sum-frequency generation (SFG) inside the ring resonator of a singly-resonant optical parametric oscillator (SR-OPO) scheme. The effect of the resonator specifications on the efficiency of such SFG–OPO configuration is modeled and simulated by solving the coupled equations in a normalized from which describe simultaneous SFG and OPO interactions inside two individual phase-matched nonlinear crystals. Numerical simulation is carried out using real practical values associated with experimental results that are recently reported by Devi et al. (Opt Express 21:24829–24836, 2013). We found very good agreement and consistent with the used experimental data, indicating the validity of our simulation. The influence of the beam waist of interacting beams on the conversion efficiency of resultant SFG–OPO radiation is investigated through introducing a magnifying factor that relates optical and physical characteristics of the designed SFG–OPO configuration. It is found that when this factor is taken equal to 3, the maximum of normalized SFG power reaches to ~0.0034 for an OPO loss of ~10 %. This is very consistent with the measured value of ~0.0030, confirming that our represented model is valid. The SFG–OPO arrangement is then optimized by extracting a mathematical formula that relates the optimum magnifying factor to the gain parameters of OPO and SFG interactions. © 2015, Springer Science+Business Media New York.
IEEE Journal of Selected Topics in Quantum Electronics (1077260X)21(1)pp. 185-192
We study thermal effects in high-power single-pass second-harmonic generation (SP-SHG) of continuous-wave (cw) green radiation in MgO:sPPLT and describe optimization of critical factors including the fundamental beam waist, position, and focusing parameter, for attainment of maximum SP-SHG efficiency. By developing a numerical model combining split-step Fourier and finite-element methods, we compute the relevant parameters for optimization of SP-SHG of a cw Yb-fiber laser at 1064 nm in a 30-mm-long crystal. Real experimental parameters and boundary conditions are used to simulate nonuniform heat distribution in the crystal under high-power operation and study its effects on SP-SHG optimization. The results indicate that for input powers exceeding a certain limit, the Boyd and Kleinman (BK) criteria for optimum focusing are no longer valid. By increasing the fundamental power to 50 W, we find that the optimum focusing parameter decreases by a factor of 4, from "\xi =2.84$ to 0.7. We also find that the maximum SP-SHG output power is always obtained for fundamental waist at the center of the crystal, and we present a general strategy for the choice of ξ at different fundamental powers. Using a top-sinked oven design, the BK focusing condition can be established even at high fundamental powers. © 1995-2012 IEEE.
Optica Applicata (00785466)45(3)pp. 355-367
We introduce the genetic algorithm for the optimization of an Yb3+-doped double-clad fiber laser based on a multi-variable scheme. The output characteristic of the laser is numerically simulated using real practical values. This is performed through solving the associated steady-state rate equation and investigating the effects of input variables such as pump and signal wavelengths and length of the fiber on the laser output. It is found that pumping of the medium around 975 nm is conducted to attain the maximum output power of ∼34.8 W, while the stability of the outcoupled power is significantly improved when pumping at 920 nm, confirming good agreement with the reported experimental results. We have also found that by using genetic algorithm base multi -variable optimization, the output power can be significantly increased by about three orders of magnitude and reaches to ∼28.5 W with optimum and shorter fiber length of ∼57.5 m. Obtained results show that based on the genetic algorithm multi-variable discipline, fiber characteristics can be optimized according to the gaining of maximum output power.
Applied Optics (21553165)54(29)pp. 8580-8586
We present a theoretical model that describes a multisegment oven configuration to compensate for the Gouy phase shift in the tight focusing regime. The model is provided to obtain maximum efficiency of the second-harmonic wave. Numerical simulations are performed based on the experimental setup employing an MgO:sPPLT nonlinear crystal. It is shown by the simulation that the three-segment oven is potentially capable of reaching the highest efficiency by canceling the effect of Gouy phase when the temperature of each segment is so optimized that the phase-matching condition is re-established. It is found that by using a multisegment oven scheme with optimized temperature, the highest attainable second-harmonic generation (SHG) efficiency exceeds about 4.4% at a confocal parameter of 3.32. Moreover, it is shown that for long crystals with large confocal parameter, the only way to attain the maximum efficiency is by using multisegment ovens. The results indicate that when the number of segments is changed from one to nine, the confocal parameter can be varied from 3.90 to 5.85. In the candidate three-segment oven, by applying active control on the temperature of each segment, for a certain combination of segment lengths, ∼4% improvement in the SHG efficiency is achieved. This results in a relatively large increase in the acceptance temperature bandwidth of the crystal of up to 2 times, reflecting a comparatively large enhancement in the second-harmonic power stability and efficiency. © 2015 Optical Society of America.
Applied Physics B: Lasers and Optics (09462171)118(2)pp. 219-229
The performance of an optical sensor that employs an unbuffered polydimethylsiloxane (PDMS)-cladding fber optic is demonstrated for the sensitive detection of CO2 gas in the near-infrared region for around 1.57 µm using the apodized 2f/1f wavelength modulation spectroscopy method. The permeability and diffusion characteristics of the PDMS fber have been theoretically examined and numerically simulated. The results of the simulation are verifed by an experimental setup containing a DFB laser source and 5-m-coiled unbuffered PDMS fber placed in a pre-vacuumed cell flled with about 980 ± 10 Torr of pure CO2 gas. A minimum detectable absorption of ~0.9 × 10-4 is measured, corresponding to a detection sensitivity of ~4.5 × 10-11 cm_1/Hz1/2. The effect of the scaling k-factor on the apodized signal is subsequently studied, showing close agreement between the simulation and experimental results. © Springer-Verlag Berlin Heidelberg 2014
Chinese Physics B (16741056)24(2)
To reduce the walk-off angle of the extraordinary third-harmonic ultraviolet wave at 355nm generated by type II KTiOPO4 and type I β-BaB2O4 optical crystals, and the Gaussian output beam of a Q-switched Nd:YAG laser, a simple theoretical model was developed based on a rotatable BK7 plate of variable thickness. By rotating the plate up to 35° along the beam direction, we reduced the walk-off angle up to ∼ 13%. The same phenomenon is predicted by the model, confirming the performance of the model. It is found that, due to the walk-off effect, the intensity profile of the third-harmonic generation beam is slightly degraded. To compensate for the observed phenomena and further reduce the walk-off, we used a combination of a convex lens and an axicon to transform the beam profile of the interacting fundamental and second-harmonic generation waves to the zero-order Bessel-Gaussian form. As a result, the walk-off is decreased to ∼48.81mrad, providing ∼30% relative reduction. By using the same BK7 plate rotated up to 35° along the third-harmonic beam direction, the walk-off angle is further reduced to 38.9mrad. Moreover, it is observed that the beam profile of the emerged Bessel-Gaussian third-harmonic generation beam remains unchanged with no degradation. © 2015 Chinese Physical Society and IOP Publishing Ltd.
Optical and Quantum Electronics (discontinued) (03068919)46(12)pp. 1547-1559
We report a new method to measure amplitude small signal gain of a chopper base Q-switched Nd:YAG laser using saturation pulse width effect of nanosecond relaxation oscillation (RO) spikes. Formation of such RO pulses is theoretically modeled and simulated by attributing a loss function to the internal Q-switch chopper and solving the standard rate equations using real practical values associated with a laboratory Q-switched cw Nd:YAG high-power laser. It is found that at a certain chopper frequency, pulse width of the simulated RO spikes is saturated and tends to a minimum value. The performance of the represented model is verified by experimental measurements through generating nanosecond RO spiked using a frequency-variable chopper inside the cavity of the laboratory Nd:YAG laser. We observed that at a certain input pump power of 180 and 290 W, when the frequency of Q-switch chopper is respectively raised up to 5.8 and 12 kHz, oscillation of single RO pulse is established. We also found that by further increasing of the chopper frequency up to 18 kHz, FWHM pulse width of the RO fundamental pulse is saturated toward ~163 ns. Moreover, at this frequency of chopper and input pump powers of 270 and 180 W, saturated FWHM RO pulse widths of 170 and 330 ns are measured, respectively. Obtained results show good consistency between theoretical model and experimental measurements. This saturation effect, is used to obtain amplitude small signal gain of the utilized Q-switched Nd:YAG laser as 3 and 0.8 at the above input pumping levels, respectively. At the same time by using Findlay–Clay approach the small signal gain is measured to provide a quantitative comparison with the above results. From the final results we found excellent agreement between the presented method and Findlay–Clay approach. © 2014, Springer Science+Business Media New York.
IEEE Journal of Selected Topics in Quantum Electronics (1077260X)20(5)
We present a theoretical model which describes the effects of thermal load distribution on single-pass second harmonic generation (SP-SHG) of high-power continuous-wave (cw) radiation in MgO:sPPLT nonlinear crystal to provide green output at 532 nm. Numerical simulations are performed based on real practical values and actual operating conditions associated with a recent SP-SHG experiment, generating 10 W of cw green radiation using a Yb-fiber laser. The model is used for four oven configurations to simulate the implications of thermal effects on SH power. The observed asymmetric feature of the phase-matching curves, particularly at higher fundamental powers up to 50 W, are characterized and explained by considering the generation of heat due to crystal absorption. The concept of optical path difference (OPD) is introduced to study the formation of thermal lens and its effects on the displacement of focal point inside the thermally loaded crystal. We further study the dependence of the SH power on the different oven schemes by increasing the input fundamental power up to 50 W. It is found that a top-sinked oven design is the optimum configuration for achieving maximum SHG efficiency without saturation. Comparison of the simulation results with experimental data confirms the validity of the theoretical model. © 1995-2012 IEEE.
Optics InfoBase Conference Papers (21622701)2014
We report a systematic study of thermal effects in high-power single-pass SHG in the presence of absorption, and propose an optimum heating configuration for the crystal to minimize thermal lensing at various fundamental power levels. © 2014 Optical Society of America.
Optics and Laser Technology (00303992)56pp. 436-442
We report the generation of a mid-infrared (MIR) difference-frequency radiation between 4.76 μm and 4.86 μm rang inside AgGaS2 nonlinear crystal using a V-shaped external cavity configuration fabricated around pump source. The effect of using such V-shaped scheme on tuning characteristics of a commercial diode laser have been studied and experimentally presented. More than ~11 nm continuous tuning is obtained for the emerging original beam through rotating a blazed grating over a certain angular range with a measured accuracy of 1.4 μm/degree. Such characterized V-shaped external cavity is used as pump source in a DFG experimental setup. An appreciable extension in generated MIR-DFG bandwidth is observed when the intracavity grating is rotated over ~600 μm in opposite directions. The performance of such extended V-shaped external cavity base MIR-DFG radiation is demonstrated as trace detection of P(16) and P(18) rovibrational lines of 1-0 fundamental band of carbon monoxide (12C16O) molecule with and without using V-shaped cavity. © 2013 Elsevier Ltd.
Applied Physics B: Lasers and Optics (09462171)110(3)pp. 425-431
We report the generation of mid-infrared pulsed radiation between 2.2 and 3 μm range using a singly-resonant optical parametric oscillator (SR-OPO) based on a 40-mm-long crystal of periodically-poled LiNbO3 (PPLN) pumped by mechanically Q-switched pulses from a Nd:YAG laser, obtained by chopping the beam inside the laser resonator over a 1-10 kHz duty cycle. An appreciable reduction in pulse width as well as the number of relaxation oscillation pulses of the Nd:YAG pump laser is observed when the frequency of the Q-switch chopper is increased up to 10 kHz. Sub-nanosecond relaxation oscillation pulses of about 170-210 ns duration are generated under the width of the idler envelope varying from 4.6 to 8.55 μs. The same behavior is observed for the signal wave. A maximum extraction efficiency of 22 % is obtained for the idler, corresponding to 785 mW of output power at 10 kHz. The tuning of the signal and idler beams were performed by temperature variation of the PPLN crystal within 100-200 °C range. © 2013 Springer-Verlag Berlin Heidelberg.
Optics Express (10944087)20(25)pp. 27442-27455
We present a theoretical model on the effects of mechanical perturbations on the output power instability of singly-resonant optical parametric oscillators (SR-OPOs). Numerical simulations are performed based on real experimental parameters associated with a SR-OPO designed in our laboratory, which uses periodically-poled LiNbO3 (PPLN) as the nonlinear crystal, where the results of the theoretical model are compared with the measurements. The out-coupled power instability is simulated for a wide range of input pump powers the SR-OPO oscillation threshold. From the results, maximum instability is found to occur at an input pump power of ~1.5 times above the OPO threshold. It is also shown theoretically that the idler instability is susceptible to variations in the cavity length caused by vibrations, with longer cavities capable of generating more stable output power. The validity of the theoretical model is verified experimentally by using a mechanical vibrator in order to vary the SR-OPO resonator length over one cavity mode spacing. It is found that at 1.62 times threshold, the out-coupled idler suffers maximum instability. The results of experimental measurements confirm good agreement with the theoretical model. An intracavity etalon is finally used to improve the idler output power by a factor of ∼2.2 at an input pump power of 1.79 times oscillation threshold. © 2012 Optical Society of America.
Chinese Optics Letters (16717694)10(6)
An antiresonant ring (ARR) interferometer configuration is introduced for the characterization of a continuous wave (CW) Nd:YAG laser output. The output of the ARR device is precisely characterized to determine the gain and loss of a laboratory CW Nd:YAG laser by using the Findlay-Clay approach. The ARR arm is then experimentally arranged inside the cavity of an arranged high power side-pumped CW Nd:YAG laser. A coated beam splitter with 50-50% reflectivity at normal incidence is placed inside the cavity to provide a wide range of reflectivity from 0 to 100%. This is performed by a rotatable stage and tilting the beam splitter by 10° with the steps of 0.05. By changing the input electrical power of the laser pump the variation of the output laser power is monitored for 20 individual reflectivity of ARR arm. Average pump threshold power of about 180 W is obtained. With the help of the derived equations and obtained threshold power, small signal gain and loss associated with the emerging beam is estimated. It is verified that the former is very dependent to the input parameters. Laser efficiency is also measures 5.6% which is quite comparable with the reported values. © 2012 Chinese Optics Letters.
Applied Physics B: Lasers and Optics (09462171)108(2)pp. 261-268
In the present article, the thermal distribution inside the gain medium of a passively Q-switched microchip laser is modeled and simulated for actual practical values associated with an available microchip laser constructed in our laboratory. The effects of the non-uniform heat distribution on the spectral properties of the output laser beam have been investigated and simulated with the variation of diode-pump power and pulse repetition rate. It is observed that the gain bandwidth as well as Optical Path Difference (OPD) values of the propagating pulses are significantly decreased, while the Nd:YAG chip is cooled down to a certain value. The validity of the utilized model is checked by setting and characterizing the spectral properties of a fabricated laboratory microchip laser under different heating conditions. It is verified that when the temperature of the gain material is changed by an electronically controlled Peltier device, the spectrum of the output laser beam can be switched between single- and dual-mode situations. This physical character has shown good agreement between the presented model and obtained experimental results. ©Springer-Verlag 2012.
Optica Applicata (00785466)41(4)pp. 897-909
In this paper, we consider properties of cosh-squared-Gaussian beam passing through ideal and apertured fractional Fourier transforms (FRFT) systems. We use Collins integral formula and the fact that a hard aperture function can be expanded into a finite sum of complex Gaussian functions. These studies indicate that the normalized intensity distributions with FRFT order are periodic. The variation period is 2 and is independent of the impact of aperture.
Iranian Journal of Physics Research (16826957)11(3)
A novel compact fiber-coupled NIR system based on a DFB diode laser source is employed as a portable and sensitive gas sensor for trace detection of combustion pollutant molecules. We demonstrate the performance of such an NIR gas sensor by tracing the absorption lines of CO and CH4 using 2f-WMS technique at moderate temperature of T ∼ 600°C in the recuperator channel of an industrial furnace provided by Mobarakeh steel company. This measurement shows the excellent sensitivity of the applied NIR gas sensor to the on-line and in-situ monitoring of such molecular species.
Chinese Optics Letters (16717694)9(7)
In this letter, a thin slab of glass is used as Fabry-Perot etalon inside a cavity of a continuous wave (CW) Nd:YAG laser to change the behavior of its output longitudinal modes. The slab etalon is used as tuning element when it turns around the laser cavity axis. Two simultaneous longitudinal modes with a relatively wide tuning range from 5.83 MHz to 0.02 THz are obtained when the Nd:YAG laser is operated at moderate output power of about 120 mW. The mode structure of this configuration is modeled and simulated. Computer-generated diagrams are also presented schematically and compared with the experimental results. © 2011 Chinese Optics Letters.
Optics and Laser Technology (00303992)43(5)pp. 956-959
We have reported a modified V-shaped external cavity, which is constructed around a commercial diode laser operating at a center wavelength of λ=785 nm by adding a new coated glass plate with about 50% reflectivity to the cavity. This allows simultaneous dual-wavelengths operation in the vicinity of Δνmin=0.18 THz to Δνmax=0.22 THz, which can be used as laser source for terahertz generation either for semiconductor devices or nonlinear schemes. © 2010 Elsevier Ltd. All rights reserved.
